Prior to launch, and continuing through the early part of the mission, flight controllers on the ground were monitoring an anomaly within the electrical power generating Fuel Cell #2. Despite troubleshooting, the anomaly persisted and Fuel Cell #2 was shut down.

Although the Shuttle had 3 fuel cells, any one of which was enough to power the mission, the problems with Fuel Cell #2 violated several Mission Flight Rules and the mission was ended early – landing after just shy of 4 days, out of a planned 16 day mission.

The Mission Flight Rules provide guidelines when making difficult decisions during space operations. Having such a set of rules, thought out carefully and thoroughly on the ground, before the circumstance arises is a long-standing safety practice.